1. Field of the Invention
The present invention generally relates to magnetic field sensitive devices, such as Hall effect sensing devices. More particularly, this invention relates to the assembly and packaging of integrated circuits including a Hall effect sensor die and magnet to reduce the minimum obtainable distance between the sensing device and a ferromagnetic object being sensed.
2. Description of the Related Art
Magnetic sensing devices which can detect the presence of a ferromagnetic object in the vicinity of the sensing device have been widely used. Such sensing devices typically utilize a magnetic field and employ sensing apparatus that detect changes in the strength of a magnetic field. Magnetic field strength is defined as the magnetomotive force developed by a permanent magnet per the distance in the magnetization direction. As an example, an increase in the strength of a magnetic field, corresponding to a drop in the reluctance of a magnetic circuit, will occur as an object made from a high magnetic permeability material,such as iron, is moved toward the magnet. Magnetic permeability is the ease with which the magnetic lines of force, designated as magnetic flux, can pass through a substance magnetized with a given magnetizing force. Quantitatively, it is expressed as the ratio between the magnetic flux density (the number or lines of magnetic flux per unit area which are perpendicular to the direction of the flux) produced and the magnetic field strength, or magnetizing force Because the output signal of a magnetic field sensing device is dependent upon the strength of the magnetic field, it is effective in detecting the distance between the sensing device and an object within the magnetic circuit. The range within which the object can be detected is limited by the flux density, as measured in gauss or teslas.
Where it is desired to determine the speed or rotational position of a rotating object, such as a disk mounted on a shaft, the object is typically provided with surface features that project toward the sensing device, such as teeth. The proximity of a tooth to the sensing device will increase the strength of the magnetic field. Accordingly, by monitoring the output of the sensing device, the rotational speed of the disk can be determined by correlating the peaks in the sensor's output with the known number of teeth on the circumference of the disk. Likewise, when the teeth are irregularly spaced in a predetermined pattern, the rotational position of the body can be determined by correlating the peak intervals with the known intervals between the teeth on the disk.
One prominent form of such a sensing device is a Hall effect sensor. A Hall effect sensor relies upon a transverse current flow that occurs in the presence of a magnetic field. The Hall effect sensor is primarily driven by a direct current voltage source having electrodes at both ends of the Hall effect sensor, creating a longitudinal current flow through the sensor's body. In the presence of a magnetic field, a transverse current is induced in the sensor which can be detected by a second pair of electrodes transverse to the first pair. The second pair of electrodes can then be connected to a voltmeter to determine the potential created across the surface of the sensor. This transverse current flow increases with a corresponding increase in the magnetic field's strength.
The Hall effect sensor is mounted within and perpendicular to a magnetic circuit which includes a permanent magnet and an exciter. The exciter is a high magnetic permeability element having projecting surface features which increase the strength of the magnet's magnetic field as the distance between the surface of the exciter and the permanent magnet is reduced. Typically, the exciter will be in the form of a series of spaced teeth separated by slots, such as the teeth on a gear. The exciter moves relative to the stationary Hall effect sensor element, and in doing so, changes the reluctance of the magnetic circuit so as to cause the magnetic flux through the Hall effect element to vary in a manner corresponding to the position of the teeth. With the change in magnet flux there occurs the corresponding change in magnet field strength, which increases the transverse current of the Hall effect sensor.
With the increasing sophistication of products, magnetic field sensing devices have also become common in products that rely on electronics in their operation, such as automobile control systems. Common examples of automotive applications are the detection of ignition timing from the engine crankshaft and/or camshaft, and the detection of wheel speed for anti-lock braking systems and four wheel steering systems. For detecting wheel speed, the exciter is typically an exciter wheel mounted inboard from the vehicle's wheel, the exciter wheel being mechanically connected to the wheel so as to rotate with the wheel. The exciter wheel is provided with a number of teeth which typically extend axially from the perimeter of the exciter wheel to an inboard-mounted magnetic field sensor. As noted before, the exciter wheel is formed of a high magnetic permeability material, such as iron, such that as each tooth rotates toward the sensor device, the strength of the magnetic field increases as a result of a decrease in the magnetic circuit's reluctance. Subsequently, the magnetic circuit reluctance increases and the strength of the magnetic field decreases as the tooth moves away from the sensing device. In the situation where a Hall effect device is used, there will be a corresponding peak in the device's potential across the transverse electrodes as each tooth passes near the device.
A common shortcoming of magnetic field sensing devices is their output's dependence upon the distance between the exciter and the sensing device, known as the air gap. More specifically, as the air gap increases, the maximum output range of the device decreases thus decreasing the resolution of the output and making it more difficult to accurately analyze the device's output. The output of a Hall effect device is directly proportional to the strength of the magnetic field, and therefore is sensitive to the air gap at low strength magnetic fields.
Conventionally, the air gap is defined as the distance between the exciter and the outer surface of the package containing the sensing device. An "effective air gap" may be described as the distance between the exciter and the sensing device itself. As can be seen in FIG. 1, the prior art magnetic field sensors 10 typically include a permanent magnet 14 and sensing device 16 encapsulated in a package 18. However, this type of packaging is unsuited for harsh environments, particularly that of an automobile. As a result, such packaged sensing devices are further enclosed in an additional housing (overmold) 20 which affords protection from moisture and dirt. Accordingly, while the sensing device's air gap 22, the distance between the exciter and the sensing device's package may be unchanged, the sensing device's effective air gap 24, the distance between the exciter and the sensing device itself, may be increased significantly. Thus, while improving the life of the sensing device, a particularly significant shortcoming to this approach is the decrease in the peak magnetic field strength as a tooth passes in proximity to the sensing device due to the larger effective air gap. In addition a variety of steps are required to assemble the numerous components of this assembly. Still another problem is that it is desirable to have the sensing device 16 as close as possible to the magnet 14 because the magnetic field decreases as a function of air gap. Being closer allows the use of a smaller or lower energy product magnet.
Thus, it would be desirable to provide a packaging scheme for a magnetic field sensing device, such as a Hall effect device, that would provide reliable protection from the environment while also avoiding an excessive increase in the effective air gap between the sensing device and the exciter, reduce the number of assembly steps, and allow the sensing device to be as close as possible to the magnet.